صحيح البخاري

Narrated `Abdullah bin Mas`ud:

I said, "O Allah's Messenger! Which is the biggest sin?" He said, "To set up rivals to Allah by worshipping others though He alone has created you." I asked, "What is next?" He said, "To kill your child lest it should share your food." I asked, "What is next?" He said, "To commit illegal sexual intercourse with the wife of your neighbor."